HADOOP-15247. Move commons-net up to 3.6.

Contributed by Steve Loughran.

hadoop-project/pom.xml

@@ -612,7 +612,7 @@
       <dependency>
         <groupId>commons-net</groupId>
         <artifactId>commons-net</artifactId>
-        <version>3.1</version>
+        <version>3.6</version>
       </dependency>
       <dependency>
         <groupId>javax.servlet</groupId>